Project Summary Overdose of acetaminophen (APAP), the popular analgesic and antipyretic agent, is the most common cause of acute liver failure (ALF) in the Western world. Treatment options for APAP-induced ALF are extremely limited. Most ALF patients are treated with N-acetyl cystein, a precursor of glutathione, which works only if delivered within hours after APAP overdose. The only other therapy is liver transplantation, which is complicated by scarcity of donor organs, life long immunosuppressant use and exorbitant cost. There is a critical need to develop novel therapies for APAP-induced ALF. Recent studies using animal models and patient samples have demonstrated that timely stimulation of innate liver regeneration is associated with better outcomes including transplant free survival in APAP-induced ALF. These studies indicate that without proper liver regeneration, APAP-induced acute liver injury can develop into ALF and result in death. These studies underscore the therapeutic potential of liver regeneration following APAP-induced ALF. However, the mechanisms of liver regeneration after APAP overdose are not completely known. In the previous cycle of this grant, we investigated the role of canonical Wnt-?-catenin signaling in liver regeneration after APAP overdose. Interestingly, our preliminary studies have uncovered a novel and paradoxical role for Hippo Kinase pathway and its downstream regulator yes associated protein (Yap) in pathogenesis of APAP-induced ALF. Our studies, performed using hepatocyte specific Yap knockout (Yap-KO) mice, indicate that Yap activation after APAP overdose is associated with delayed regeneration after APAP overdose. Deletion of Yap resulted in enhanced liver regeneration leading to faster recovery after APAP overdose. Mechanistically, deletion of Yap in hepatocytes resulted in faster activation of ?-catenin, which stimulated the faster liver regeneration after APAP administration. Furthermore, we observed a faster and augmented pro-regenerative inflammatory response driven by higher expression of serum amyloid A and CXCL1, both ?-catenin targets in hepatocytes, in the Yap- KO mice. Finally, our preliminary studies indicate that Yap and its target gene connective tissue growth factor (CTGF) could be used as prognostic markers in APAP-induced ALF. These studies will explore an unconventional and novel role of Yap in drug induced acute liver failure and subsequent liver regeneration. Successful completion of these studies will have substantial impact on clinical management of APAP overdose because they will identify Yap as a new target for regenerative therapies for APAP-induced ALF patients.